The present invention relates to an electroslag remelting or refining process for the removal of impurities from a metal, particularly steel.
An arrangement is known which permits the utilization of the advantages of fusion-electrolysis, hence the discharge of undesired ions, in electroslag remelting by means of at least one self-consuming electrode. Such an arrangement is described in U.S. Pat. No. 3,571,475 to Holzgruber, et al, issued Mar 16, 1971. This arrangement permits the control of the direction of metallurgical reactions and the removal of undesired elements, like sulfur, oxygen etc., to an increasing extent from the melt. It makes use of the principle that the slag, which is present in the molten state in the remelting system, is to a great extent ionized due to the joulean heat generated during the passage of the current, and the individual ions can be moved in the slag by superposing a direct current.
In electroslag-remelting or refining with alternating current, the polarity of self-consuming electrode and liquid ingot sump changes periodically according to the supply frequency. When the electrode passes through the positive half wave, for example, the ingot sump forms the negative pole. By means of at least one non-melting auxiliary electrode, which consists preferably of graphite, a d.c. component is introduced by way of rectifiers into the remelting system so that both the electrode and the ingot have a positive (or negative) potential difference relative to the auxiliary electrode. In this manner, it is possible to start a fusion electrolysis which results, on the one hand, in an ionic migration to the auxiliary electrode, and on the other hand, in migration to the self-consuming electrode and to the sump. Depending on the polarity of the auxiliary electrode, certain ions are discharged on the auxiliary electrode and deposited after reactions with the electrode graphite or atmospheric oxygen. Chemical reactions with the slag are also possible.
Such a known electroslag remelting system is connected to an a.c. transformer having one pole connected to the melting electrode and having a second pole connected to the bottom plate, and thus to the ingot. This single-phase transformer supplies the a.c. energy required for the electroslag-remelting. The current direction between ingot and consuming electrode changes with the a.c. frequency. By introducing non-melting auxiliary electrodes into the slag, it is possible to superpose a d.c. component on this a.c. flux, the direct current flowing to or from the auxiliary electrode system. A second pole then supplies the melting-electrode or the ingot. The d.c. voltage is generated by a rectifier. In order to pole the self-consuming or melting electrode to the auxiliary electrode, the ingot phase is connected with the auxiliary electrode system by way of a rectifier so that the positive or the negative half wave becomes effective, depending on the direction of the current through the rectifier. According to the same system, the melting electrode phase is connected with the auxiliary electrodes. A switch is provided which is able to reverse the polarity of the auxiliary electrodes. A rheostat can be connected to regulate the auxiliary electrode current. The regulation of the current can be effected, on the one hand, by the resistance and, on the other hand, by the position of the auxiliary electrode system in the slag relative to the ingot and consuming electrode.
A substantial part of the known arrangement consists thus of one (or several) non-melting auxiliary electrodes dipping into the slag, which electrodes consist preferably of graphite. In practical operation, the service life of these non-melting auxiliary electrodes is of great importance, since melting times of 100 hours or more must be expended for the production of large ingots, and chemical reactions between the auxiliary electrode and the slag must be avoided. Particularly, the carbonization of the remolten ingots by the graphite of the auxiliary electrodes in the above described system is a serious disadvantage.
Another disadvantage of the known arrangement is that the direct current for feeding the auxiliary electrodes is taken from the a.c. circuit and supplied by way of rectifiers to the auxiliary electrode system. Since one half wave of the alternating current is rectified, non symmetries occur in the transformer in the primary side. In addition, the desired electrolytic reactions only take place above a certain threshold voltage. The superposed voltage must therefore be regulatable to a great extent. In the case of the known arrangement, the voltage can only be varied, however, by varying a rheostat.